Many techniques are presently known for maximizing the volume of oil produced by an oil well. One of these, for example, is called "production logging." Production logging generally refers to the process of lowering a "tool string" into a producing oil well that has been cased and perforated. The tool string may include a number of well known devices for performing various functions, such as perforating the well casing, sealing perforations in the well casing, pumping petroleum from the well, measuring characteristics of fluids in the well, and the like.
Petrophysicists are often interested in measuring characteristics of different fluids in the well, at different depths, to determine which depths of the well are producing oil, and the rate at which they are producing. Typically, perforations are made in the well casing at different depths to permit oil to flow into the wellbore from the surrounding strata. Although it is advantageous to create these perforations at depths corresponding to oil-bearing strata, these perforations are sometimes made at depths where a mixture of oil and water is located, or where water exists alone. In some cases, perforations are made at depths that initially produce a great deal of oil but eventually produce more and more water and less and less oil, due to depletion of the oil reserves at that depth. If it is determined that a certain depth of the well is non-producing, or is producing mostly saline water ("brine"), some remedial work is performed upon that depth of the well. For example, the perforations in the casing at that depth may be plugged to stop production. Then, other more productive depths of the well may continue producing. Moreover, new production may be initiated by perforating the casing at other, untapped depths of the well.
It is therefore an important function of production logging to measure the ratio of water to oil at different depths inside the well casing. Such a production log indicates whether the fluid flowing into the well at a particular location is oil or water. In most cases, the flow of water into the well does not add any value, but increases the cost of production. The indication of water to oil allows the well operator to take remedial action to plug the perforation in the well producing mostly water and otherwise control the well to maximize depletion of oil in the formation.
One known method of determining water and oil content in a fluid sample or fluid stream is to measure the resistivity of the fluid. Such a resistivity measurement is a measure of the resistance per unit length of a quantity of the fluid having a unit cross-section. The oil and underground water usually have very different resistivities. The underground salty water is more conductive than oil. By measuring the resistivity of the fluid, one can easily distinguish between oil and water. Indeed, resistivity measurement is one of the most important well logs (in most cases, the most important log) used by petrophysicists to evaluate the earth formation.
Known methods of measuring resistivity of fluid in a production well include various electronic probes and the like. Such instruments require the use of electronic equipment in the harsh environment of the well. A source of electrical power must be provided to such electronic equipment. It is also know to use sample chambers to help recognize the presence of water. With this arrangement, flowing fluids are directed into a partially enclosed volume within the logging tool where the fluids are coupled to measurement electrodes. Although this approach may be useful in some applications, it may be difficult to ensure that a representative sample will be obtained, due to the tendency of oil-water mixtures to separate in low-turbulence environments.